Credit: © 2008 ACS

The addition of particles to microfluidic devices could create new possibilities for the targeted delivery of chemical cargo, and the ability to precisely control their movement is of obvious importance. Now Pietro Tierno and colleagues at the University of Barcelona and the University of Sheffield have designed1 a simple method for propelling linked magnetic microparticles in solution by controlling their rotation.

Two iron-oxide-doped polystyrene microparticles of different size are linked together through strands of complementary DNA that are anchored to the particles' surfaces — creating an anisotropic, paramagnetic 'doublet'. When placed in solution above a glass plate, the doublet floats approximately 200 nm above the glass surface due to electrostatic interactions. A magnetic field that precesses around an axis parallel to the glass plate can then be applied, causing the doublet to rotate in a way such that the smaller particle precesses in a plane perpendicular to the adjacent surface — like an oar — inducing the doublet to move in one specific linear direction.

In bulk solution the particle would just rotate in a stationary position, displaying only Brownian motion, but the movement seen by Tierno and co-workers arises when the doublet is close to the glass plate. As the particle rotates the viscous friction is higher when the smaller particle is closer to the glass plate than when it is on the opposite side of the larger particle — thus the small particle can 'grip' the plate without physically touching it, driving it forward. The speed and direction of the doublet can be manipulated by controlling the frequency and orientation of the precessing magnetic field respectively.